From European Patent Letter No. 068 301 a steam generator is known, with which in a first fluidized-bed cooler an evaporator heating surface with a by-pass line in parallel thereto and in a second fluidized-bed cooler only heating surfaces for reheating are arranged. With this arrangement the heating of the reheating steam is controlled by the bed temperature which is in turn influenced by the solids stream recycled via said fluidized-bed cooler.
Further from the periodical "Modern Power Systems", December/January 1984/85, p. 57, a steam generator is known with which two evaporator heating surfaces and two reheater heating surfaces are arranged in separate fluidized-bed coolers. By-pass lines are not provided; an injection cooler is, however, interconnected between the two reheater heating surfaces. By means of this injection cooler and the solids stream fed to the fluidized-bed cooler the temperature control of the reheater outlet temperature is provided.
With the known steam generators a simple influence on the heat balance in the combustion loop: fluidized-bed combustor, separator, fluidized-bed cooler, fluidized-bed combustor is not possible. In case of large and quick load variation there is unsufficient control behavior with respect to keeping a substantially constant reheater outlet temperature and a substantially constant flue gas inlet temperature in the waste-heat boiler pass.
From the German periodical "VGB-Kraftwerkstechnik", Vol. 5, p. 373, FIG. 12, a steam generator is known, with which two fluidized-bed coolers are provided, in each of which three heating surfaces of the same type are arranged.